Skin is the largest organ of the body, and perhaps one of the most important. It serves many different functions, including acting as a barrier, maintaining homeostasis, sensory systems, immunologic systems, as well as exocrine functions. The skin maintains these functions via cells in its many different layers, shown below. Most superficially lies the epidermis (or epithelium) made up of what are characteristically considered “”skin cells””. Below this is a thin layer of matrix called the basal lamina (or basement membrane) atop which the epithelium sits. Farther down is the dermis, a layer of connective tissue (collagen), and even lower is the hypodermis (not labeled) which contains largely fat tissue. Various specialized cells as well as free nerve endings are found throughout both the epidermis and dermis and contribute to the many sensations perceived at the level of the skin.

The cells attached to the basal lamina (sometimes called basal cells) do so by a connecting structure called a hemidesmosome. Unlike desmosomes, which connect two cells together and are also found in the epithelium, hemidesmosomes link one cell to the extracellular matrix of another. As such, basal cells – the cells of the epidermis lining the basal lamina – are linked to the extracellular matrix of the basal lamina by hemidesmosomes. Both desmosomes and hemidesmosomes act to resist shear forces. The cells in the epidermis also contain many varieties of the protein keratin which forms intermediate filaments, also important in maintaining structural integrity. Junctional epidermolysis bullosa (JEB) is a genetic condition that results in mutations of hemidesmosomes. A genetic tree is shown below. Red indicates the presence of blistering (the phenotype resulting from JEB). White indicates lack of the phenotype. Squares indicate males, and circles indicate females.

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